Did you ask the buyer to measure her dog before you sent out the belly band?  That's the first thing I do when a buyer is unsure of the size.  Not much help to you in this case, however it's up to the buyer to get the item back to you if she's not happy.  Make sure you keep all messages in the ebay system and I would flag them all so that ebay will be able to read them in the event of less than positive feedback.:-)